The present invention generally relates to communication systems and more particularly to a transmit/receive antenna system for generating a single conical scanned monopulse beam for accurately pointing the system at a single Ku-band communications satellite without interfering with adjacent satellites.
Numerous communications satellites are now in geo-stationary orbit around the earth to facilitate global communications. Such satellites are located at a fixed position relative to the earth. These satellites are often located very close to one another in terms of circumferential alignment relative to the earth. In fact, many communications satellites are located about two degrees from one another.
One advantage of closely locating such geo-stationary communications satellites is that many satellites become available for use by earth bound (or near earth bound) antenna systems. Unfortunately, one disadvantage of placing satellites so close to one another is that miscommunication due to interference with adjacent satellites may occur. The potential for interference with adjacent satellites increases if the satellite is communicating with an earth based antenna system which is moveable rather than fixed.
Antenna systems which are designed to be moveable relative to the earth while communicating with a geo-stationary communications satellite include those placed on moving platforms such as airplanes, ships, and automobiles. The most common type of such mobile antenna systems is a receive-only antenna system which has no transmit capability. Advantageously, since no signal is sent from a receive-only system to the satellite, receive-only systems do not interfere with adjacent satellites in geo-stationary orbit.
Unfortunately, receive-only systems have limited capabilities. For example, receive-only systems are mainly for used for viewing direct television and dish satellite television signals. Modern communication needs commonly require both a receive signal and a transmit signal.
To provide the required transmit signal while maintaining the ability to receive signals, a transmit/receive antenna system is necessary. Unfortunately, conventional transmit/receive systems that transmit signals to geo-stationary satellites conically scan both the receive signal and the transmit signal. Conically scanning the transmit signal causes the resulting beam to be transmitted at a conical angle relative to the line of sight of the antenna system. Since the transmit beam is transmitted outboard of the line of sight, interference with adjacent satellites can occur.
In view of the foregoing, it would be desirable to provide a transmit/receive antenna system for a moving platform that does not scan the transmit beam so that the system could accurately track a desired communications satellite without interfering with adjacent satellites.
The above and other objects are provided by an antenna system including a feedhorn, main reflector, sub-reflector, and frequency selective member. The sub-reflector includes a symmetrical reflecting surface coaxially aligned with the feedhorn and main reflector. The frequency selective member includes a non-symmetrical reflecting surface coaxially aligned between the feedhorn and sub-reflector. The frequency selective member transmits signals having a first frequency from the feedhorn to the sub-reflector. The sub-reflector reflects these signals to the main reflector. The frequency selective member reflects signals having a second frequency from the main reflector to the feedhorn.
In operation, the non-symmetrical reflecting surface of the frequency selective member rotates relative to the main reflector and feedhorn. The non-symmetry and rotation of the reflecting surface of the frequency selective member reflects receive signals at a small angle relative to the line of sight of the antenna system. The symmetry of the reflecting surface of the sub-reflector reflects transmit signals axially symmetric relative to the line of sight. In this way, the transmit/receive system only conically scans the receive signals.
Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention.